For various kinds of information transmission/control, the communication cable is used in a vehicle and a communication device and in a room or the like. As this communication cable, an optical fiber for optical communications or a conductor for electric communications is used such that a single-core wire or a multicore wire is covered on its outer circumference directly with an armor or in a loose state. In another case, a protecting tube for an armor to cover the communication wire is laid in advance in the cable shape, and the communication wire may later be inserted, if necessary, into the protecting tube.
The communication wire may be deteriorated in the transmission characteristics by a bend or side pressure. It is, therefore, necessary to prevent the communication wire from being excessively bent. As the communication cable 10 using the optical fiber for preventing the transmission loss due to the bend, there has been known in the related art the construction which is provided with annular corrugations on the armor as referred to in patent publication JP-A-11-223752.
FIG. 18 is a diagram showing the communication cable which is provided with a bend preventing coating, as disclosed in JP-A-11-223752. In FIG. 18: reference numeral 1 designates an optical fiber; numeral 2 a sheath; and numeral 3 a bend preventing coating. The optical fiber 1 is formed of the optical fiber (as usually called the “plastic fiber”), in which a core portion made of an acrylic resin or polycarbonate resin is enclosed with a similar resin having a lower refractive index than that of the core portion.
This optical fiber 1 is covered on its outer circumference with the sheath 2 of an extensible polyethylene resin or vinyl chloride resin, which is covered on its outer side with the bend preventing coating 3 made of a resin like that of the sheath 2. The bend preventing coating 3 is corrugated by forming a number of annular slits along the circumference of its surface. The communication cable thus constructed can be prevented from being bent to a predetermined or larger angle, because the adjoining convex portions of the bend preventing coating 3 come into contact with each other when the communication cable is bent.
However, the bend preventing coating 3 thus far described has such a structure that the corrugations are formed in the coating portion made of the identical extensible polyethylene resin, vinyl chloride resin or the like.
Therefore, a little difference is made in the force required for the bend before and after the contact of the adjoining convex portions. When the communication cable is bent, therefore, there is a danger that the bend is proceeded over the predetermined or smaller diameter by an inertia or the like. When the adjoining convex portions contact, moreover, the coating is deformed so that a sufficient bend preventing effect cannot be attained. If the corrugated bend preventing coating 3 is made of a hard resin so as to reduce the deformation of the convex portions, therefore, the convex portions are whitened when the communication cable is repeatedly bent, so that they are disadvantageously cracked.
The present invention has been conceived in view of the background thus far described and has an object to provide a communication cable and a communication wire protecting tube, which can be easily bent to a predetermined bend radius by a low force but which requires such a higher force for the bend of a smaller diameter that it may be hard to bend.